This invention relates to circuit breakers. More particularly, it is concerned with low voltage circuit breakers for controlling low and moderate power electrical circuits.
A circuit breaker for use in controlling electrical circuits typically has a set of contacts, one fixed and one movable, and a toggle, or overcenter mechanism, which is manually operated to close and open the contacts. A circuit breaker also includes an overload mechanism for tripping the circuit breaker and opening the contacts automatically when the electrical current through the circuit breaker exceeds certain predetermined conditions.
Typically circuit breakers of this type include an overcenter spring which is connected between a movable contact arm carrying the movable contact and a trip arm which is held latched in a set position under normal operating conditions. The contact arm pivots about a pivot point which is shiftable by means of a manually operated handle to place the pivot point on one side of the overcenter spring whereby the spring causes the contacts to be closed, or to place the pivot point on the other side of the overcenter spring whereby the spring causes the contacts to be open. Under overload conditions a current sensitive mechanism releases the latched trip arm and the overcenter spring moves the trip arm from the set to a tripped position. Movement of the trip arm to the tripped position shifts the position of the overcenter spring with respect to the pivot point and the force of the overcenter spring in this position causes the contacts to open.
Typically the overload mechanism includes a bimetallic element which has one end fixed with respect to the circuit breaker housing. The other end of the bimetallic element engages the trip arm but is free with respect to the housing. Under overload conditions the bimetallic element deflects and the free end moves to release the trip arm which moves from the set to the tripped position thereby opening the contacts. Under very high current conditions, such as during a short circuit, the bimetallic element heats rapidly and deflects almost instantaneously to release the trip arm.
Under overload conditions of relatively small magnitude, however, the bimetallic element is designed to deflect slowly so that the overload mechanism trips only after a substantial time interval which is related to the overload current. Small variations in the dimensions of the breaker components can result in a significant effect on the reaction time of the overload mechanism. These variations may be due to such factors as differences in coefficients of thermal expansion, wear over the life of the breaker, or external mechanical stresses.